Administration of the 5-HT1A receptor agonist, 8-hydroxy-2(di-n- propylamino) tetralin (8-0HDPAT), to rats produced dose-dependent decreases in food intake and hypothermia, increases in plasma prolactin and corticosterone, and a decrease in plasma growth hormone. Pretreatment with the nonselective S-HT receptor antagonist, metergoline, did not affect 8-OHDPAT-induced decreases in food intake but attenuated the prolactin release, and also potentiated 8-0HDPAT-induced hypothermia. Long-term (21 days) treatment with the monoamine oxidase (MAO) type A inhibiting antidepressant, clorgyline, but not the tricylic antidepressants, clomipramine and imipramine, attenuated the hypothermic response to 8-OHDPAT. In another study, short-term (2-6 days) or long-term (21-25 days) treatment with clorgyline potentiated fenfluramine- induced suppression of food intake but did not affect fenfluramine- induced suppression of locomotor activity. On the other had, long- term but not short-term imipramine treatment attenuated fenfluramine-induced decreases in food intake but not locomotor activity. These results indicate that various agents effective in different types of affective disorders exert differential modulatory influences on serotonergic mechanisms regulating food intake, locomotor activity, temperature, and neuroendocrine changes in vivo. In a separate series of studies, the fawn-hooded (FN) rat strain was found to be significantly less sensitive to the food intake suppressant effects of m-chlorophenylpiperazine (m-CPP, a 5-HT agonist), 8-OHDPAT (a selective 5-HT1A agonist), and fenfluramine (a 5-HT releasing agent); locomotor suppressant effects of m-CPP and prolactin responses to m-CPP than either Wistar or Sprague- Dawley (SD) rat stains. Isolated FH rats gained significantly less body weight relative to isolated Wistar or SD rats. These finding demonstrate that FH rats, a strain with a peripheral platelet storage pool disorder, also posses altered central nervous system serotonergic function.